Boron carbide cermet structural materials with high flexure strength at elevated temperatures

ABSTRACT

Prepare boron carbide-aluminum structural composites by infiltrating molten aluminum or aluminum alloy into a porous preform that is either unbaked or baked at a temperature of up to 1800° C. to form a densified cermet and then heat treat the cermet in air or an oxygen-containing atmosphere to form a dense outer surface layer of aluminum oxide. The resulting structural cermets can withstand prolonged exposure to temperatures above the melting point of aluminum without suffering undue degradation of physical properties such as flexure strength.

BACKGROUND OF THE INVENTION

This invention relates generally to boron carbide-aluminum ceramic-metalcomposites (cermets) suitable for use as structural parts and theirpreparation. This invention relates particularly to cermets yieldingstructural parts that can withstand prolonged exposure, in air, attemperatures of 625° Centigrade (° C.) or above. This invention relatesmore particularly to such cermets that have a surface layer of aluminumoxide (Al₂ O₃)and their preparation.

U.S. Pat. No. 4,605,440 discloses a process for preparing boroncarbide-aluminum composites. The process includes a step of heating apowdered admixture of aluminum (A1) and boron carbide (B₄ C) at atemperature of 1050° C. to 1200° C. The process depletes of most of theAl by forming a mixture of several ceramic phases that differ from thestarting materials.

U.S. Pat. No. 4,702,770 discloses a method of making a B₄ C--Alcomposite. The method includes a preliminary step of heating B₄ Cpowder, in the presence of free carbon, at temperatures ranging from1800° C. to 2250° C. This step reduces reactivity of B₄ C with moltenAl. During this step, the B₄ C particles from a rigid network that,after infiltration by molten Al, substantially determines mechanicalproperties of resulting composites.

U.S. Pat. No. 4,718,941 discloses a method of making metal-ceramiccomposites from ceramic precursor starting constituents. Theconstituents are chemically pretreated, formed into a porous precursorand then infiltrated with molten reactive metal. The chemicalpretreatment alters starting constituent surface chemistry and enhancesmolten metal infiltration. Ceramic precursor grains, such as B₄ Cparticles, held together by multiphase reaction products formed duringinfiltration constitute a rigid network that substantially determinesmechanical properties of a resultant composite.

In preparing a B₄ C--Al cermet via infiltration of molten Al into aporous B₄ C preform, reactivity depends primarily upon reaction time.This poses a major problem because chemistry changes as a front ofmolten Al moves into the preform. The change in chemistry results in acermet with large differences in microstructure. A portion of thepreform that first comes into direct contact with infiltrating metaldiffers significantly, in terms of amount of reaction phases andreaction phase morphology, from a portion that comes into direct contactwith infiltrating metal at or near completion of infiltration. Thesedifferences lead to residual stresses that promote cracking of resultingcermets.

Post-infiltration heat treatments of a cermet, typically conducted in avacuum or in argon at temperatures exceeding that at which metalcomponents of the cermet melt, lead to two additional problems. First,they promote reductions in free metal content and make initialdifferences even more pronounced. As a result, cracking increases inseverity. Second, the heat treatments result in increased porosity,especially at or near external surfaces of the cermet. The porosityresults because molten Al does not wet B₄ C or mostboron-aluminum-carbon (B--Al--C) phases at temperatures of less than900° C. As a result, surface tension forces metal toward externalsurfaces of the cermet, thereby creating a zone of porosity.

SUMMARY OF THE INVENTION

An aspect of the invention is a method for making a B₄ C--Al structuralcomposite. The method comprises sequential steps: a) infiltrating moltenaluminum or aluminum alloy into a porous B₄ C preform to form adensified B₄ C--Al cermet; and b) heat treating the densified cermet inthe presence of air or an oxygen-containing atmosphere at a temperatureof at least about 625° C. for a period of time sufficient to yield (1) amicrostructure characterized by isolated B₄ C grains or clusters of B₄ Cgrains surrounded by a multiphase matrix that comprises aluminumborides, aluminum borocarbides and free metal, and (2) a dense surfacelayer of aluminum oxide, the surface layer being substantially free ofcarbon. The preform is either unheated greenware or greenware that hasbeen heat treated at a temperature of 1800° C. or less.

Another aspect is a B₄ C--Al structural composite having a dense surfacelayer of Al₂ O₃ that is substantially free of carbon, and amicrostructure that comprises isolated B₄ C grains or clusters of B₄ Cgrains surrounded by a multiphase matrix that comprises aluminumborides, aluminum borocarbides and free metal, the metal being Al or anAl alloy,

DETAILED DESCRIPTION

Boron carbide is a ceramic material characterized by high hardness andsuperior wear resistance. It is a preferred starting material for theprocess aspect of the invention, particularly when it is present as aporous preform.

A second preferred starting material for the process aspect is Al or anAl alloy that has an Al content of greater than 80 percent by weight (wt%), based upon alloy weight. Aluminum is typically used in cermets toimpart toughness or ductility to the ceramic material.

The process aspect of the invention begins with a porous ceramic preformor greenware article prepared from B₄ C powder by conventionalprocedures. These procedures include slip casting a dispersion of theceramic powder in a liquid or applying pressure to powder in the absenceof heat. The powder desirably has a particle diameter within a range of0.1 to 10 micrometers (μm). Ceramic materials in the form of plateletsor whiskers may also be used.

The preform may contain a ceramic filler material in an amount of fromgreater than 0 to 50 wt %, based upon total preform weight. The preformpreferably contains from 70 to about 95 wt % B₄ C and from about 5 to 30wt % ceramic filler. The percentages are based upon total preform weightand total 100%. The ceramic filler material is at least one of titaniumdiboride, titanium carbide, silicon boride, Al₂ O₃ and silicon carbide.

The porous preform is desirably baked at a temperature of at least 1400°C. prior to infiltration. Baking should continue for at least 15minutes, desirably at least 30 minutes and preferably about two hours ormore. If the preform has an infiltration distance (depth of metalpenetration) of less than 0.3 inches (about 0.8 centimeters (cm)),baking before infiltration is not necessary because infiltration issubstantially complete before large microstructural differences develop.Although not necessary, a heat treatment or baking at a temperature of100°-1300° C. may be used for infiltration distances of less than 0.3inches (about 0.8 cm).

If the infiltration distance is 0.3 inches (about 0.8 cm) or larger, theporous preform must be baked at a temperature of at least 1400° C.,preferably within a range of from 1400° C. to less than 1800° C. Thetemperature is maintained for a period of from about 15 minutes to about5 hours, preferably from about 1 to about 3 hours. This heat treatment(baking) slows down chemical reaction kinetics during infiltration withmolten metal in comparison to infiltration of a porous preform that hasnot been heat treated, especially for those with large infiltrationdistances (up to 12 or even 13 inches). Baking therefore allowsproduction of large parts without excessive chemical reaction andattendant microstructural differences between various areas of aresulting cermet part.

Baking temperatures that are below 1400° C., even within a range of1350° C. to less than 1400° C., do not slow down chemical reactionkinetics enough to yield generally uniform microstructures wheninfiltration distances exceed 0.8 cm, particularly when such distancesare much greater than 0.8 cm.

Baking temperatures of 1800° C. or more prior to infiltration lead touniform microstructures, but promote formation of excess carbon. Duringa post infiltration heat treatment, the excess carbon reacts withresidual unreacted metal to form amounts of aluminum carbide (Al₄ C₃)that exceed one wt %, based on total cermet weight. The presence of Al₄C₃ is highly undesirable as it adversely affects physical properties ofcermet parts resulting from a post-infiltration heat treatment asdescribed herein.

Cermet parts resulting from infiltration of baked B₄ C preforms withinfiltration distances greater than 0.8 cm have more uniformmicrostructures than those resulting from infiltration of unbaked B₄ Cpreforms with the same infiltration distance. Although they have a moreuniform microstructure, the cermet parts are not suitable for extendeduse at high temperatures because of residual unreacted metal. In orderto overcome this deficiency, the resulting cermet parts must besubjected to an additional (post-infiltration) heat treatment. Theadditional heat treatment occurs at a temperature within a range of fromabout 660° to about 1250° C., preferably from about 660° to about 1100°C., more preferably from about 800° to about 950° C., in the presence ofair or some other oxygen-containing atmosphere. This heat treatment hasa duration sufficient to allow slow reactions between residual unreactedmetal and B₄ C or B--Al--C reaction products or both. The reactionspromote some reduction of free (unreacted) metal amounts and developmentof large parts with a generally uniform microstructure that issubstantially free of cracks induced by residual stresses.

The additional heat treatment also promotes formation of a dense andsubstantially continuous Al₂ O₃ surface layer that serves severalpurposes. First, it protects the heat treated cermet from furtheroxidation. Second, it slows movement of liquid metal from internalregions of the cermet to its surface portions. Third, it puts the cermetpart under compressive pressure thereby improving its flexure strength.Fourth, it provides chemical stability and improves corrosion resistancesince Al₂ O₃ is much more chemically stable than either Al or B₄ C.

Post-infiltration heat treatments at temperatures outside the range offrom about 660° to about 1250° C. yield unsatisfactory results.Temperatures of less than about 660° C. lead to reactions that are tooslow to reduce residual unreacted metal levels to 15 wt %, based ontotal cermet weight, or less. The residual unreacted metal levelspreferably fall within a range of from about 2 to about 8 wt %, based ontotal cermet weight. In addition, oxidation occurs so slowly at thesetemperatures that the Al₂ O₃ surface layer does not have sufficientthickness to accomplish the foregoing purposes. Temperatures in excessof 1250° C. lead to formation of undesirable amounts of aluminum carbide(Al₄ C₃) and an Al₂ O₃ surface layer that is not uniform in thickness.

The post-infiltration heat treatment has a duration that typicallyranges from 1 to about 100 hours, desirably from about 10 to about 75hours, and preferably from about 25 to about 75 hours. Cermet partswherein the infiltration distance is 0.8 cm or less must be subjected tothe same post-infiltration heat treatment if they are to be suitable forsuch high temperature use. A duration in excess of 100 hours increasesproduction costs, but yields no substantial additional changes inmicrostructure over those occurring at 100 hours. A duration of lessthan 1 hour does not allow enough change to occur in the microstructureand produces an Al₂ O₃ layer that is too thin to fulfill the foregoingpurposes.

Structural composites prepared by the process aspect of the inventionare characterized by presence of (1) a bulk microstructure containingisolated B₄ C grains or clusters of B₄ C grains surrounded by amultiphase matrix and (2) an Al₂ O₃ surface layer. The matrix comprisesat least one, preferably at least two, of aluminum borides, aluminumborocarbides and free metal, wherein the metal is either aluminum or analuminum alloy. The composites comprise from about 40 to about 75 wt %B₄ C grains, from about 20 to about 50 wt % aluminum borides andaluminum borocarbides and from about 2 to about 8 wt % aluminum oraluminum alloy, all percentages being based upon composite weight andtotaling 100%. The aluminum borides and aluminum borocarbides areselected from the group consisting of AlB₂₄ C₄, Al₃ B₄₈ C₂, Al₄ BC,AlB₂, and AlB₁₂. The aluminum borides and borocarbides are desirablyAlB₂₄ C₄ and AlB₂, preferably with a ratio of AlB₂₄ C₄ /AlB₂ that iswithin a range of from about 10:1 to about 1:5. The latter range is morepreferably from about 10:1 to about 2:1.

Phases such as AlB₂₄ C₄ and AlB₂ are more oxidation resistant andtherefore more desirable than either B₄ C or Al₄ BC. When large clustersof AlB₂₄ C₄ are present close to a surface of a structural composite,the post-infiltration heat treatment results in a thinner Al₂ O₃ layerthan when Al₄ BC is close to the surface. Alternating between AlB₂₄ C₄clusters and Al₄ BC produces an Al₂ O₃ layer of variable thickness.Maximizing AlB₂₄ C₄ content near the surface leads to a generallyuniform Al₂ O₃ layer that is more desirable than an Al₂ O₃ layer ofvariable thickness.

The thickness of the Al₂ O₃ surface layer depends largely upon heattreatment temperature and time. For example, a heat treatment at 600° C.for 200 hours produces an Al₂ O₃ layer having a thickness of only about2 μm. Heat treatments of the same duration at 800° C., 900° C. and 1000°C. produce respective Al₂ O₃ layer thicknesses of 12-15 μm, 50-75 μm and100 μm. The Al₂ O₃ layer actually consists of at least two sublayers, anouter sublayer of relatively large Al₂ O₃ fibers that have a thicknessof 0.1 to 0.2 μm and an intermediate zone or sublayer of Al₂ O₃ fibersthat have a thickness of about 100Å (1000 nanometers). The intermediatesublayer is disposed between the outer sublayer and an interface betweenthe Al₂ O₃ layer and the cermet part or substrate upon which the Al₂ O₃layer is formed.

Chemical analysis of the Al₂ O₃ surface layer shows that the layercontains a small amount of boron (B), but no carbon (C). The amount of Bvaries from about 2 to about 6 percent by volume (vol %), based upontotal layer volume. An amount of 2 vol % is typically found in the outersublayer whereas an amount of 6 vol % is more typical of theintermediate sublayer, particularly close to the interface. The chemicalanalysis does not reveal any boron oxide (B₂ O₃), so the B is mostlikely present as boron-containing Al--B--C and Al--B ceramic phasessuch as Al₃ B₄₈ C₂.

The structural composites of the invention have a number of potentialend uses, many of which build upon the high temperature flexure strengthand Young's modulus (stiffness) retention of such composites. Thecomposites can be used as automotive parts, particularly as engine partssuch as valves, and as structural parts that require high specificstrength at temperatures up to 1000° C.

The composites of the invention are surprisingly useful as structuralmaterials that can withstand prolonged (hundreds of hours) exposure totemperatures in excess of 660° C., the melting point of aluminum, butless than 1000° C. Pure B₄ C has poor oxidation resistance. Attemperatures above 450° C. in the presence of air, B₄ C starts tooxidize forming excessive amounts of B₂ O₃. This leads, in turn, to arapid reduction of flexure strength from a room temperature (23° C.)strength of 350 megapascals (MPa) to a strength at 800° C., afteroxidation in air for a period of 200 hours, of about 100 MPa. At roomtemperature, Al metal has a flexure strength of about 70 MPa. At 660°C., Al melts and its strength reduces to 0 MPa. The structuralcomposites of the invention unexpectedly have flexure strengths at roomtemperature that equal or exceed that of B₄ C at room temperature. Thestructural composite flexure strengths typically range from 460- 500 MPaat room temperature, 380-450 MPa at 900° C. and 200-250 MPa at 1300° C.

The structural composites have a hardness (measured by Vickersindentation method with 14.4 kg load) that is between 1350 kg/mm² to1700 kg/mm². A hardness of 1350 kg/m² is achievable after a heattreatment of 10-15 hours. A longer heat treatment of at least 25 hours,typically 25-50 hours, leads to a hardness of 1700 kg/m².

The structural composites have physical properties other than hardnessand flexure strength that are noteworthy. They have a fracture toughnessof between 6 and 7 MPa·m^(1/2) and a Young's modulus that ranges between320 and 360 gigapascals (GPa). The composites have an electricalconductivity that is similar to pure aluminum.

The structural composites of the invention are light in weight with adensity that ranges from about 2.7 grams per cubic centimeter (g/cm³)(for a 100% B₄ C preform) to less than 3.2 g/cm³ when B₄ C is combinedwith a ceramic filler material.

When a ceramic filler material is used to prepare the structuralcomposites of the invention, the filler is present in the compositeseither as isolated grains or as part of the clusters of B₄ C grains. Theamount of ceramic filler material is from about 1 to about 25 vol %,based upon total composite volume.

The following examples further define, but do not limit the scope of theinvention. Unless otherwise stated, all parts and percentages are byweight.

EXAMPLE 1

B₄ C (ESK specification 1500, manufactured by ElektroschemeltzwerkKempten of Munich, Germany, and having an average particulate size of 3μm) powder was dispersed in distilled water to form a suspension. Thesuspension was ultrasonically agitated, then adjusted to a pH of 7 byaddition of NH₄ OH and aged for 180 minutes before being cast on aplaster of Paris mold to form a porous ceramic body (greenware) having aceramic content of 69 vol %. The B₄ C greenware was dried for 24 hoursat 105° C. The greenware sizes were 120×120×10 millimeters (mm) (thintiles) and 120×120×16 mm (thick tiles).

Pieces of the greenware were used as is (unbaked), baked at 1300° C. for120 minutes, baked at 1400° C. for 120 minutes, baked at 1800° C. for 60minutes or baked at 2200° C. for 60 minutes. All baking and sinteringtook place in a graphite element furnace. The baked greenware pieceswere then infiltrated with molten Al (a specification 1145 alloy,manufactured by Aluminum Company of America that is a commercial gradeof Al, comprising less than 0.55% alloying elements such as Si, Fe, Cuand Mn) with a vacuum of 100 millitort (13.3 Pa) at 1180° C. for 120minutes to provide cermet pieces.

Cermet pieces prepared from the thin tiles were all quite uniform fromtop to bottom even though some differences were noticeable. As such, thebaking temperature did not have a significant impact uponmicrostructure.

Cermet pieces prepared from the thick tiles had nonuniformmicrostructures that varied from bottom (closest to infiltrating metal)to top (farthest from the infiltrating metal) in amount of B--Al--Cphases and in phase morphology. The bottoms had a microstructure ofequiaxed AlB₂ and Al₄ BC with less than 2 vol % free Al. The tops had amicrostructure of AlB₂ and Al₄ BC in a form resembling comparativelylarge cigars (50-100 μm in length) in an Al matrix. The amount of freeAl ranged between 5-15 vol %. The microstructural differences werespecially visible in cermets resulting from greenware pieces baked at1300° C.

Infiltration times in excess of 2 hours tend to yield nonuniformmicrostructures with some cracking of the cermet pieces. For example,infiltration times of 3 and 5 hours produce cracking and splitting incermets resulting from greenware baked at 1300° C. This problem may bereduced or eliminated by baking the B₄ C greenware at a temperature inexcess of 1300° C.

                  TABLE I                                                         ______________________________________                                                Part Uniformity                                                                             Vickers                                                         (Bottom to Top)                                                                             Hardness* (kg/mm.sup.2)                                 B.sub.4 C                                                                           Bake    10 mm    16 mm    Before  After                                 Bake  Time    Thick    Thick    Heat    Heat                                  Temp  (min-   Green-   Green-   Treat-  Treat-                                (°C.)                                                                        utes)   ware     ware     ment    ment                                  ______________________________________                                         20    0      Uniform  Nonuniform                                                                             1300    1550                                  1300  120     Uniform  Nonuniform                                                                             700     1420                                  1400  120     Uniform  Uniform  450     1700                                  1800  120     Uniform  Uniform  480     1750                                  2200   60     Uniform  Uniform  450     1030                                  ______________________________________                                         *14.4 Kg load                                                            

Table I shows that by baking B₄ C preforms at or above 1400° C., uniformmicrostructures can be obtained. Baking above 1400° C. is believed topassivate B₄ C surfaces and slow down chemical reaction kinetics. Thisresults in uniform parts which have an amount of unreacted free metal.These parts are not, however, suitable for use in applications thatrequire prolonged high temperature (less than 1000° C.) exposure.Materials suitable for such applications should have (i) ceramic-ceramicinterfaces free of metal to provide strength and (ii) a protective outerlayer to reduce or minimize oxidation of the B₄ C. The foregoing partscan meet this criteria if they are subjected to a post-infiltrationheat-treatment in air.

As shown in Table I, a post-infiltration heat treatment at 690° C., inair for 50 hours provided an increase in hardness for all cermet pieces.The data in Table I show that, at least from a hardness point of view,B₄ C greenware baked at temperatures below 1800° C. yields better heattreated cermets than greenware baked at temperatures above 1800° C. Thegreenware that was baked at 1400° C. and 1700° C. results in cermetswith uniform microstructures and high hardness values. The data in TableI further show that green B₄ C and B₄ C baked below 1400° C. produceuniform and hard parts when limited to small sizes (<10 mm verticalmetal flow). As vertical metal flow distances exceed 10 mm in green B₄ Cand B₄ C baked below 1400° C., hardness remains relatively high, butresulting parts exhibit nonuniform microstructures and cracking.

EXAMPLE 2

Pieces of greenware were prepared and infiltrated with or without bakingas in Example 1. Chemical analysis of the infiltrated greenware pieceswas completed using an MBX-CAMECA microprobe, available from Cameca Co.,France. Crystalline phases were identified by X-ray diffraction (XRD)with a Phillips diffractometer using CuKa radiation and a scan rate of2° per minute. The amount of Al present in the infiltrated greenware wasestimated based upon differential scanning calorimetry (DSC). All of thegreenware pieces were then heated from the melting point of Al (660° C.)to 900° C. over a period of one hour before 3× 4×45 mm specimens fromone-half of the pieces were subjected to Flexure Strength testing usinga four-point bend test (ASTM C1161) at 900° C. The samples weremaintained in air at that temperature for 15 minutes before they werebroken. Upper and lower span dimensions were 20 and 40 mm, respectively,and the specimens were broken using a crosshead speed of 0.5 mm/min.Specimens from the other pieces were subjected to an additional heattreatment for 25 hours in air at 690° C. before they were heated againto 900° C. over a period of one hour and broken in Flexure Strengthtesting.

The data presented in Table II show that the heat treatment history ofgreenware prior to infiltration has a marked influence upon FlexureStrength of the resultant B₄ C/Al cermets. The data also show,particularly for Samples A and H, that metal content alone does notdetermine strength at elevated temperatures. The strength at hightemperature is also affected by ceramic phases formed duringinfiltration. Samples G, H and I have the highest flexure strengthvalues prior to a post-infiltration heat treatment. This may be due tofast chemical reaction kinetics in conjunction with a sufficient amountof B₄ C. The data further show that the post-infiltration heat treatmentleads to an increase in flexure strength. Nonetheless, only some of thesamples reached a flexure strength level of >350 MPa.

Similar results are expected with other compositions and processconditions, all of which are disclosed herein.

                  TABLE II                                                        ______________________________________                                        Phase Chemistry                                                                                                       Flexure                                                Green-                 Strength                                               ware                   after                                        Composi-  Bake     Residual      Heat                                  Sample tion (wt  Temper-  A1     Flexure                                                                              Treat-                                Iden-  % B.sub.4 C/wt                                                                          ature    Content                                                                              Strength                                                                             ment                                  tification                                                                           % A1)     (°C.)                                                                           (wt %) (MPa)  (MPa)*                                ______________________________________                                        A      85/15     2200     10     188    --                                    B      80/20     2200     15     266    290                                   C      70/30     2200     25     170    --                                    D      80/20     1400     15     180    430                                   E      70/30     1400     25     170    450                                   F      64/36     1400     30      52    380                                   G      80/20      20       5     400    --                                    H      75/25      20      10     400    410                                   I      70/30     1300     15     383    390                                   J      45/55     1300     40      52    --                                    ______________________________________                                         -- means not measured; *means 25 hours in air at 690° C.          

EXAMPLE 3

Cermet (B₄ C--Al) samples having respective initial B₄ C and Al contentsof 75 vol % and 25 vol % were prepared by baking B₄ C greenware at 1300°C. for 30 minutes and infiltrating the greenware with the same Al alloyas in Example 1 for 60 minutes at 1150° C. The greenware, prior toinfiltration, was in the form of tiles measuring 120×120×10 mm. Afterinfiltration, the tiles were ground into 4×3×45 mm bars. The bars weredivided into 4 groups. The first group (Group A) of samples were used asinfiltrated, the second (Group B) was heat treated at 800° C. in argonfor 100 hours, the third (Group C) was heat treated in air at 800° C.for 2 hours, the fourth (Group D) was heat treated in air at 800° C. for100 hours. The samples were all subjected to Flexure Strength testing asin Example 2 save for changing the temperatures (Table III) at whichsamples were broken.

                  TABLE III                                                       ______________________________________                                        Group/  Flexure Strength (MPa) at                                             Temp-   Various Temperatures (°C.)                                     erature 20     200    400  600  700  800  900  1100                           ______________________________________                                        A       520    510    460  320  300  240  200  --                              B*     --     --     --   330  310  300  250  --                             C       --     --     --   330  350  380  400  290                            D       --     510    460  430  440  --   440  340                            ______________________________________                                         -- means not measured; *means not an example of the invention            

The data in Table III show that heat treatment in argon, an inert gas,does not enhance high temperature strength in the same manner as heattreatment in air. Both heat treatments produce B--Al--C phases, but onlythe heat treatment in air produces an oxidation layer that effectivelystops metal flow towards cermet outer surfaces. Strength increases as afunction of heat treatment time in air. Although samples from Group D(heat treated in air at 800° C. for 100 hours) have the same strength atroom temperature as those of Group A (no heat treatment), they have amuch higher strength above 600° C.

EXAMPLE 4

Cermet (B₄ C--Al) samples having respective initial B₄ C and Al contentsof 70 vol % and 30 vol % were prepared by baking B₄ C greenware at 1400°C. for 120 minutes and infiltrating the greenware with the same Al alloyas in Example 1 for 60 minutes at 1160° C. The greenware, prior toinfiltration, was in the form of tiles measuring 120×120×10 mm. Afterinfiltration, the samples were cut into bars measuring 4×4×45 mm. Thebars were then subjected to heat treatments in air at varioustemperatures for varying lengths of time, all of which are shown inTable IV. After completion of the heat treatment, the cross-sections cutfrom the bars were polished using, in succession, 30, 15, 6, 1 and 0.25μm diamond pastes and subjected to scanning electron microscopy (SEM) todetermine thickness of the resulting Al₂ O₃ layer. The data reported inTable IV for each cross-section represent an average of 40 measurements.

                  TABLE IV                                                        ______________________________________                                        Heat         Al.sub.2 O.sub.3 Layer thickness (μm) at                      Treatment    Heat Treatment Times (hours)                                     Temp (°C.)                                                                          50     100        200  500                                       ______________________________________                                        600          --     --          2   --                                        700          --      6         --   --                                        800          12     12         11   18                                        850          13     16         20   22                                        900          18     27         50   --                                        1000         --     16         70   --                                        1100         80     --         350  --                                        ______________________________________                                    

The data presented in Table IV demonstrate that oxide layer thicknessincreases with increasing heat treatment times and temperatures. Similarresults are expected with other times and temperatures as describedherein. The data point for 1000° C. and 100 hours is believed to be inerror based upon data trends for heat treatments of 200 hours duration.

EXAMPLE 5

Cermet (B₄ C--Al) samples having respective initial B₄ C and Al contentsof 70 vol % and 30 vol % were prepared by baking B₄ C greenware at 1300°C. for 30 minutes and infiltrating the greenware with the same Al alloyas in Example 1 for 60 minutes at 1160° C. The tiles were ground intotest bars measuring 4×3×45 mm bars. All test bars were heat treated inargon for 100 hours. After the argon heat treatment, groups of the testbars were subjected to an additional heat treatment in air at 800° C.for lengths of time as shown in Table V. Oxide layer thicknesses,measured as in Example 4, and flexure strengths measured at 800° C. asin Example 2, are also shown in Table V.

                  TABLE V                                                         ______________________________________                                        Heat                    Oxide                                                 Treatment      Flexure  Layer                                                 Time in Air    Strength Thickness                                             (Hours)        (MPa)    (μm)                                               ______________________________________                                         0             285      0                                                      2             286      2                                                      5             295      3                                                     10             292      3                                                     25             315      5                                                     50             450      13                                                    100            290      15                                                    ______________________________________                                    

The data presented in Table V clearly show the effect of oxide layer.Fifty hours of heat treatment in air provided the maximum flexurestrength. Similar results are expected with other compositions andprocesses as disclosed herein.

EXAMPLE 6

Cermet (B₄ C--Al) samples having respective initial B₄ C and Al contentsof 70 vol % and 30 vol % were prepared by baking B₄ C greenware at 1425°C. for 2 hours and infiltrating the greenware with the same Al alloy asin Example 1 for one hour at 1160° C. The samples were in the form oftiles measuring 120×120×16 mm. The hardness after infiltration was 720kg/mm² and flexure strength at room temperature was 650 MPa. The tileswere subjected to a post-infiltration heat treatment in air at thetemperatures and for the times shown in Table VI. After 50 hours, bothtiles had a residual metal content, estimated as in Example 2, of 5-8vol %. The data in Table VI represent hardness measurements resultingfrom the heat treatment.

                  TABLE VI                                                        ______________________________________                                        Hardness Measurements (kg/mm.sup.2)                                           Heat                                                                          Treatment Heat Treatment Times (Hours)                                        Temp (°C.)                                                                       5         10     15     25   50                                     ______________________________________                                        700       --         871   --     1314 1674                                   800       1200      1230   1330   1565 1720                                   ______________________________________                                    

The data in Table VI demonstrate that hardness can be improved bypost-infilitration heat treatments in air. Similar results are expectedwith other combinations of process conditions as disclosed herein.

EXAMPLE 7

Cermet (B₄ C--Al) samples having respective initial B₄ C and Al contentsof 70 vol % and 30 vol % were prepared by baking B₄ C greenware at 1800°C. for 1 hour and infiltrating the greenware with the same Al alloy asin Example 1 for one hour at 1160° C. The samples, in the form of tilesas in Example 6, were subjected to a post-infiltration heat treatment inair for 50 hours at a temperature of 690° C. The flexure strengths atroom temperature and 800° C. were, respectively, 640 MPa and 410 MPa.The Hardness was 1680 kg/mm². The heat-treated cermets or structuralcomposites also had a fracture toughness of 6.4 MPa·m^(1/2) and a creepdeformation, under a 250 MPa load at 900° C. for 100 hours, of 0.22 mm.The composites had an Al₂ O₃ layer that was 10 μm thick and amicrostructure that consisted of B₄ C grains surrounded by AlB₂, Al₄ BCand AlB₂₄ C₄ as major ceramic phases.

EXAMPLE 8

Cermet (B₄ C--Al) samples having respective initial B₄ C and Al contentsof 68 vol % and 32 vol % were prepared by baking B₄ C greenware at 1300°C. for one hour and infiltrating the greenware with the same Al alloy asin Example 1 for two hours at 1180° C. The samples, in the form of tilesas in Example 6, were ground into 40 bars measuring 4×3×45 mm. One half(20) of the bars were subjected to a post-infiltration heat treatment inair for 25 hours at a temperature of 800° C. The other 20 bars were notheat treated. Ten bars from each set of bars were tested for flexurestrength.

All bars were immersed in a 1N hydrochloric acid (HCl) bath for 30minutes, one hour and five hours. HCl was used as it is known to attackAl and Al-containing borides and carbides. The bars were then removedfrom the bath, washed, dried and tested for flexure strength to evaluatestructural damage. The bars that had been subjected to apost-infiltration heat treatment retained 88.1% of their originalstrength after 30 minutes of immersion, 86% after one hour of immersionand 83% after five hours of immersion. The other bars (nopost-infiltration heat treatment) retained only 79.4% of their originalstrength after 30 minutes, 64% after one hour and 53% after 5 hours.This example shows that formation of a dense layer of Al₂ O₃ on thesurface of B₄ C--Al cermet provides a structural composite withincreased chemical resistance in comparison to a cermet that lacks theAl₂ O₃ surface layer. The example also shows that the presence of thedense Al₂ O₃ layer allows the composites to be used in a corrosivechemical environment.

What is claimed is:
 1. A boron carbide-aluminum structural compositehaving a dense surface layer of aluminum oxide about 5 to 13 microns inthickness that is substantially free of carbon, and a microstructurethat comprises isolated boron carbide grains or clusters of boroncarbide grains surrounded by a multiphase matrix that comprises aluminumborides, aluminum borocarbides and free metal, the metal being aluminumor an aluminum alloy.
 2. A composite as claimed in claim 1, wherein theboron carbide grains constitute from about 40 to about 75 percent byvolume and the matrix comprises from about 20 to about 50 percent byvolume of aluminum borides and aluminum borocarbides and from about 2 toabout 8 percent by weight of aluminum or aluminum alloy, all percentagesbeing based upon composite volume and totaling 100 percent.
 3. Acomposite as claimed in claim 1, wherein the aluminum borides andaluminum borocarbides are selected from the group consisting of AlB₂,Al₄ BC, AlB₁₂ and AlB₂₄ C₄.
 4. A composite as claimed in claim 3,wherein AlB₂₄ C₄ and AlB₂ are present in a ratio of AlB₂₄ C₄ /AlB₂ thatis within a range of from about 10:1 to about 2:1.
 5. A composite asclaimed in claim 1, wherein the microstructure further comprises anamount of ceramic filler material, the filler material being present asisolated grains or as part of the clusters of boron carbide grains, theamount being from about 1 to about 25 percent by volume, based upontotal composite volume.
 6. A composite as claimed in claim 5, whereinthe ceramic filler material is at least one of titanium diboride,titanium carbide, silicon boride, aluminum oxide and silicon carbide. 7.A composite as claimed in claim 1, wherein the composite has a densesurface layer that consists essentially of aluminum oxide and issubstantially free of carbon and boron.